The basic reprographic process used in an electrostatographic printing machine generally involves an initial step of charging a photoconductive member to a substantially uniform potential. The charged surface of the photoconductive member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas irradiated by the light image. This procedure records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. The latent image is then developed by bringing a developer material, including toner particles adhering triboelectrically to carrier granules into contact with the latent image. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet having the toner image thereon is then advanced to a fusing station for permanently affixing the toner image to the copy sheet in image configuration. In electrostatographic machines using a drum-type or an endless belt-type photoconductive member, the photosensitive surface thereof can contain more than one image at one time as it moves through various processing stations.
The portions of the photosensitive surface containing the projected images, so-called xe2x80x9cimage areasxe2x80x9d, are usually separated by a segment of the photosensitive surface called the inter-document space. After charging the photosensitive surface to a suitable charge level, the inter-document space segment of the photosensitive surface is generally discharged by a suitable lamp to avoid attracting toner particles at the development stations. Various areas on the photosensitive surface, therefore, will be charged to different voltage levels. For example, there will be the high voltage level of the initial charge on the photosensitive surface, a selectively discharged image area of the photosensitive surface, and a fully discharged portion of the photosensitive surface between the image areas.
The approach utilized for multicolor electrostatographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in a substantially conventional manner to form a finished color copy.
As described, the surface of the photoconductive member must be charged by a suitable device prior to exposing the photoconductive member to a light image. This operation is typically performed by a corona charging device. One type of corona charging device comprises a current carrying electrode enclosed by a shield on three sides and a wire grid or control screen positioned thereover, and spaced apart from the open side of the shield. Biasing potentials are applied to both the electrode and the wire grid to create electrostatic fields between the charged electrode and the shield, between the charged electrode and the wire grid, and between the charged electrode and the (grounded) photoconductive member. These fields repel electrons from the electrode and the shield resulting in an electrical charge at the surface of the photoconductive member roughly equivalent to the grid voltage. The wire grid is located between the electrode and the photoconductive member for controlling the charge strength and charge uniformity on the photoconductive member as caused by the aforementioned fields. Control of the field strength and the uniformity of the charge on the photoconductive member are very important because consistently high quality reproductions are best produced when a uniform charge having a predetermined magnitude is obtained on the photoconductive member.
A useful tool for measuring voltage levels on the photosensitive surface is an electrostatic voltmeter (ESV) or electrometer. The electrometer is generally rigidly secured to the reproduction machine adjacent the moving photosensitive surface and measures the voltage level of the photosensitive surface as it traverses an ESV probe. The surface voltage is a measure of the density of the charge on the photoreceptor, which is related to the quality of the print output. In order to achieve high quality printing, the surface potential on the photoreceptor at the developing zone should be within a precise range. In a typical xerographic charging system, the amount of voltage obtained at the point of electrostatic voltage measurement of the photoconductive member, namely at the ESV, is less than the amount of voltage applied at the wire grid of the point of charge application.
A fundamental challenge in designing an ESV is measuring a voltage in the 1 KV range without touching the surface being measured. Commercially available devices generally work in the 30 to 50 volt range. All commercially available ESVs including the Xerox designed units (such as disclosed in U.S. Pat. No. 5,489,850, entitled xe2x80x9cBalance Beam Electrostatic Voltmeter Modulator Employing A Shielded Electrode and Carbon Fiber Conductorsxe2x80x9d are based on a null-balance feedback system.
It is the object of the present invention to achieve, a xe2x80x9cnon-floatingxe2x80x9d i.e. xe2x80x9cconnected to groundxe2x80x9d ESV. Generally, a circuit powered by a xe2x80x9cfloatingxe2x80x9d power supply, as shown in FIG. 4 is used to sense and process the modulated signal generated by a variable capacitance xe2x80x9cmodulatorxe2x80x9d or xe2x80x9cprobexe2x80x9d. This modulator interrupts the electrostatic field generated between the surface being tested and the sense electrode, thus converting the DC voltage difference between that surface and the sensing electrode into an AC signal that is proportional to the voltage difference and the capacitance coupling. The capacitance is dependent on the spacing between the electrode and the surface under test. The result is an AC signal that is both voltage and spacing dependent. This signal is then processed by additional circuitry and converted to a DC voltage which drives a xe2x80x9chigh voltage stagexe2x80x9d which is connected between ground and the floating circuit and which xe2x80x9cdrivesxe2x80x9d the floating circuit to the same voltage as that being sensed. Usually this is done by an integrating circuit in basic classical control form, as shown in FIG. 5.
The system has been xe2x80x9cnull balancedxe2x80x9d. The speed and accuracy of this processing is dependent on the xe2x80x9cgainxe2x80x9d of the system which is a function of the spacing and the modulation frequency. The common practice is to include an electronic xe2x80x9cgainxe2x80x9d adjustment to optimize the performance at the operating spacing.
Referring to FIG. 5, the system is dependent on having a high voltage output device 1; a high voltage power supply 2; a low voltage power supply 3 that floats at the voltage being measured and a low voltage power supply 4 referenced to earth ground. A cost analysis shows that a significant portion of the cost of the ESV is related to high voltage components, i.e. items 1, 2, and 3. Also, from classical control theory, the integral feedback system limits the overall speed of response to about 10 times the period of the modulation frequency. It is an object of the present invention to eliminate the need for these items.
It is also an object of the invention to utilize reliable, low cost, and potentially high precision micro sensors. For example, Polysilicon microbridges have been driven vertically and laterally as resonant microsensors. With respect to laterally driven microbridges, short displacements of a comb type drive of the type shown and described in U.S. Pat. No. 5,025,346, typically on the order of one to ten micrometers, lead to very weak sensed signals.
U.S. Pat. Nos. 6,177,800 and 5,517,123 disclose a Microelectromechanical Systems (MEMS) type noncontacting, electrostatic voltmeter (ESV) which is integrated with on-chip signal-processing circuits. The ESV works on the principle of intermittent shuttering and exposing a detector electrode to an electric field. The chopped electric field produces a small AC current in a detector circuit. Laterally driven polysilicon resonant microstructures are used as shutters in the integrated ESV. These resonant microshutters are electrostatically driven by interdigital comb fingers. See also Loconto, D. P. and Muller, R. S., xe2x80x9cHigh-Sensitivity Micromechanical Electrostatic Voltmeter,xe2x80x9d 7th International Conference on Solid State Sensors and Actuators, 1993, pp. 878-881.
An object of the present invention is an ESV that utilizes a spacing compensating system. The system incorporates measuring the amplitude of each cycle, the speed of response is essentially the period of the modulation. Therefore, the Spacing Compensating ESV significantly reduces cost and significantly improves performance relative to the speed of response.
In accordance with one aspect of the present invention, there is provided an electrostatic type voltmeter for measuring the potential on a surface, the voltmeter including a probe; a support for supporting said probe in spaced relationship with said surface, said probe having a plurality of spacing element sites thereon for measuring a distance between each of said plurality of spacing element sites and a corresponding area on said surface opposite of each said plurality of spacing element sites; a plurality of electrostatic element sites, intermixed and adjacent to said plurality of spacing element sites on said probe, for measuring a voltage between each of said plurality of spacing element sites and an area on said surface adjacent to said corresponding area opposite of each said plurality of spacing element sites. A processor for compensating an output signal of said probe in response to the measurements received from said plurality of spacing element sites and said plurality of electrostatic element sites.
An advantageous feature of the present invention is that a distance compensation technique that enables the design of a lower cost electrostatic voltmeter based on MEMS Technology. The present invention proposes the measurement of the current flowing to the capacitor created by the spacing of the ESV to the voltage surface being measured. The present invention current varies proportionally to the spacing variations which also varies the measurement voltage output. Thus, the spacing current measurement can be used to compensate for the variations in measurement voltage caused by spacing variations thereby eliminating the need for high voltage feedback and high voltage power supply.
Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings.